1. Field of the Invention
The present invention relates to a fuel cell system.
2. Description of Related Art
Conventionally, techniques for generating electric power by a fuel cell stack along with avoiding damage of a catalyst in a fuel cell have been proposed, through preventing a cell voltage from coming in the range of the oxidation-reduction proceeding potential of the catalyst. Such techniques have been developed as an approach of improving the durability of a fuel cell stack used in a fuel cell vehicle or the like (referred to Japanese Laid-Open Patent Publication No. 2007-5038).
However, according to Japanese Laid-Open Patent Publication No. 2007-5038, it is needed to increase or decrease the electric power outputted from the fuel cell stack with respect to the required electric power demanded by a power consuming load such as a traveling motor or the like, in order to set the cell voltage out of the voltage range with the oxidation-reduction potential of the catalyst. In such a case, the excess electric power is charged in a battery (or electricity storage device), while the electric power in deficiency is assisted (or supplied) by the battery.
Accordingly, the frequency of the charge/discharge processes becomes too many in order to continuously prevent the cell voltage from being set into the range with the oxidation-reduction potential of the catalyst. As mentioned above, the too many frequencies of the charge/discharge processes of the battery cause electric energy loss associated with the charge/discharge processes, resulting in the decrease in the vehicle efficiency (for example, undesirably large fuel expenses).